Gas fluidized bed terpolymerization of olefins

ABSTRACT

The present invention relates to a gas fluidized bed process for the production of copolymers having a density between 0.900 and 0.935, which process comprises copolymerization of (a) ethylene, (b) propylene and/or 1-butene, and (c) alpha-olefins comprising from 5 to 8 carbon atoms in the gaseous state, in admixture with an inert gas and optionally with hydrogen, in the presence of a catalytic system comprising a cocatalyst consisting of at least 1 organo-metallic compound of a metal of groups II and III of the Periodic Table of Elements and a special solid catalyst.

This application is a continuation of application Ser. No. 07/085,255,filed Aug. 11, 1987, which is a continuation of Ser. No. 06/810,328,filed Dec. 9, 1985, all now abandoned.

The present invention relates to a gas fluidised bed process for theproduction of copolymers of ethylene, propylene and/or 1-butene andalpha-olefins comprising from 5 to 8 carbon atoms, these copolymershaving a density between 0.900 and 0.935 and having excellent mechanicaland optical properties.

It is known that low-density polyethylene (LDPE), that is to say of adensity less than 0.940, obtained according to a process forpolymerizing ethylene by radical-forming catalysis under high pressureand at high temperature, possesses good optical properties such as goodtransparency for a slender thickness, but on the other hand hascomparatively mediocre mechanical properties, especially as regardstearing strength, perforation and impact resistance, making it difficultto obtain finished objects with a very thin wall, such as film.

It is further known that copolymers of ethylene and propylene and/or1-butene of the "linear low density polyethylene" type (LLDPE), with adensity between 0.900 and 0.940, obtained by the copolymerisation ofethylene and alpha-olefins in the presence of a catalyst of theZiegler-Natta type, generally possess mechanical properties which aresuperior to those of the LDPE's, but unfortunately they have mediocreoptical properties.

It is known that LLDPE film prepared from a copolymer of ethylene and ahigher alpha-olefin, comprising from 5 to 18 carbon atoms, such as1hexane, 4-methyl-1-pentene or 1-octene has improved optical propertiesand especially, improved transparency. Such an LLDPE, furthermore, hasthe advantage of having tearing strength, perforation and impactresistances which are notably superior to those of the LLDPE's obtainedby copolymerising ethylene, propylene and/or 1-butene.

It is also known that copolymers of ethylene, propylene and/or 1-buteneand a higher alpha-olefin comprising 5 to 18 carbon atoms, with adensity comprised between 0.900 and 0.940, not only have excellentoptical properties when these copolymers are converted into film, butalso mechanical properties which are as good as, if not better thanthose of the LLDPE's obtained by copolymerising ethylene and a higheralpha-olefin comprising from 5 to 18 carbon atoms.

These copolymers can be produced in solution in a solvent medium of aliquid hydrocarbon. The process of this type, according to which thecopolymer is produced in the form of a solution in the liquidhydrocarbon medium, involves difficult operations for recovering thesaid copolymer from the solution. Moreover, a substantial part of thecopolymer remains dissolved in the solvent, which renders the recoveryand purification operations of the said solvent difficult.

It is also possible to produce in the gas phase copolymers of ethylene,propylene and/or 1-butene and a higher alpha-olefin comprising 5 to 12carbon atoms, having a density comprised between 0.900 and 0.940. Thesecopolymers are obtained by means of a copolymerisation reaction in thepresence of a catalyst system comprising

(a) a solid catalyst prepared generally by grinding magnesium chloridein the presence of titanium tetrachloride and possibly a halogenatingagent or an electron donor agent, and

(b) an organoaluminium compound as co-catalyst.

Catalysts prepared in this manner occur in the form of a powderconsisting of particles with a broad particle size distribution. Hencesuch a catalyst cannot be used to carry out fluidised bed polymerisationreactions except as relatively low fluidisation speeds, for example ofless than 3 times the minimum fluidisation speed, in order to avoid anysubstantial loss of particles from the fluidised bed by gas entrainment.

To satisfactorily remove the heat of reaction, it is then necessary toemploy a system of mechanical agitation and/or a device forintroduction, recovery and recycling of one or more readily volatileliquids. Accordingly, a solid catalyst of this type is not very suitablefor use in a gas-phase fluidised-bed copolymerisation reactor because ofits structure and composition. In point of fact a solid catalyst of thiskind, prepared in the absence of reducing agent, comprises a tetravalenttitanium salt, deposited on the surface of the magnesium chloridesupport in a relatively small quantity, such that the atomic ratio ofthe quantity of magnesium to the quantity of titanium is equal orsuperior to 10. The result is that as soon as this solid catalyst isintroduced into the fluidised bed reactor, the copolymerisation reactionstarts up very abruptly, creating on the one hand localised runawayreactions in the fluidised bed, and on the other hand the bursting ofthe solid particles of catalyst, the particle size of which is then nolonger controlled. These localised runaway effects in the reactionproduce hot spots and generally lead to the formation of agglomeratesand reactor fouling.

Producing copolymers such as those described above in the gas phase, inthe presence of a catalyst system comprising in particular a catalystassociated with an inorganic granular support is also known. Thisinorganic granular support generally consists of refractory oxides suchas alumina or silica. The catalyst comprising a mixture of magnesiumchloride and titanium tetrachloride, previously dissolved in a solventof the electron donor compound type such as tetrahydrofuran, isdeposited or precipitated on this granular support, which confers on thesaid catalyst specific and interesting properties for the technique offluidised bed copolymerisation connected in particular with the particlesize and resistance to abrasion of the catalyst.

But it is likewise known that the dispersion of a catalyst on a granularsupport of this type is accompanied by a growth in the catalyticactivity, especially at the start-up of the copolymerisation reaction,this phenomenon also being amplified by the presence in thepolymerisation medium of comonomers, such as for example 1-butene or1-hexene. The result is localised reaction runaways, and also phenomenaof bursting of the catalyst particles, so that copolymer obtained occursin the form of a powder consisting of particles having forms reminiscentof "orange peel" and consequently this powder has a relatively low bulkdensity. This drawback may be partially avoided by dispersing thecatalyst on a sufficiently large quantity of support. Unfortunately thecopolymers obtained in this way have comparatively high contents ofinorganic residues, which is harmful to the properties.

Another drawback of using catalysts of this type in gas-phasecopolymerisation, especially in a fluidised bed, is bound up with thefact that of necessity they contain considerable quantities of anelectron donor agent which may induce "secondary" copolymerisationreactions, particularly in the presence of heavy comonomers such as1-hexene, 4-methyl-1-pentene and 1-octene, leading to the formation ofoligomers and/or copolymers with a low molecular weight and/or a highcontent of comomoners, of a very sticky nature favouring the formationof agglomerates which are prejudicial to the proper operation of afluidised bed reactor.

A process has now been found which makes it possible to avoid the aboveproblems and to obtain by gas-fluidised bed copolymerisation, copolymersof (a) ethylene and (b) propylene and/or of 1-butene and (c) at leastone alpha-olefin, comprising 5 to 8 carbon atoms, the copolymers havinga density comprised between 0.900 and 0.935, and having improvedmechanical and optical properties.

The present invention provides a gas fluidised bed process for theproduction of copolymers having a density comprised between 0.900 and0.935, this process being characterised in that it comprises acopolymerisation of (a) ethylene, (b) propylene and/or 1-butene and (c)alpha-olefins comprising from 5 to 8 carbon atoms in the gaseous statein admixture with an inert gas and optionally hydrogen, the gaseousmixture circulating from bottom to top through the fluidised bed of thecopolymer in course of formation, the copolymerisation being effected ata temperature comprised between 50° C. and 100° C. in the presence of acatalyst system comprising:

on the one hand a cocatalyst consisting of at least one organo metalliccompound of a metal of groups II and III of the Periodic Table ofElements,

on the other hand a solid catalyst of the general formula

    Mg.sub.m Me.sub.n Ti(OR.sub.1).sub.p (R.sub.2).sub.q X.sub.r D.sub.s

in which Me is an aluminium and/or zinc atom, R₁ is an alkyl groupcomprising 2 to 8 carbon atoms, R₂ is an alkyl group comprising 2 to 12carbon atoms, X is a chlorine or bromine atom, D is an electron donorcompound, the titanium being in a valency state less than the maimum,where

m is comprised between 1 and 8, preferably between 2 and 5

n is comprised between 0 and 1, preferably between 0 and 0.5,

p is comprised between 0 and 2, preferably between 0.5 and 2

q is comprised between 0.01 and 1, preferably between 0.5 and 0.8,

r is comprised between 2 and 22, preferably between 6 and 12, and

s is less than 0.2, preferably equal to 0;

the partial pressures (pp) of the various ingredients of the gaseousmixture being such that:

0.05≦pp comonomer (b)/pp ethylene ≦0.4

0.05≦pp comonomer (c)/pp ethylene ≦0.2

0≦pp hydrogen/pp ethylene ≦0.5

0.2≦pp inert gas/total pressure ≦0.8 and

0.01 Mpa≦pp comonomer (c) ≦0.1 MPa

the comonomer (b) being propylene or 1-butene or a mixture of these twoolefins, the comonomer (c) heing an alpha-olefin comprising 5 to 8carbon atoms of a mixture of these alpha-olefins.

A catalyst particularly preferred for use in the present invention isprepared by reacting at between -20° C. and 150° C., and preferablybetween 60° C. and 90° C., magnesium metal with an alkyl halide R₂ X andone or more tetravelent titanium compounds having the formula TiX_(4-t)(OR₁)_(t), wherein R₁ is an alkyl group containing 2 to 8 carbon atoms,R₂ is an alkyl group containing 2 to 12 carbon atoms, X is chlorine orbromine and t is an integer or fraction from 0 to 3.

In this case the reagents are preferably employed in molar ratios suchthat:

0.1≦TiX_(4-t) (OR₁)_(t) /Mg≦0.33

and 0.5≦(R₂)X/Mg≦8

and preferably 1.5≦(R₂)X/Mg≦5.

Another technique of preparing a solid catalyst suitable for use in theinvention consists in reacting an organomagnesium compound and acompound of titanium at its maximum valency. Preferably the solidcatalyst in this case is formed from a compound obtained by reacting, atbetween -20° and 150° C. and more especiallly between 60° and 90° C.,one or more compounds of tetravalent titanium, of the formula TiX_(4-t)(OR₁)_(t) in which X is an atom of chlorine or bromine, R₁ is an alkylgroup containing 2 to 8 carbon atoms and t is an integer or fractioncomprised between 0 and 3, and an organo magnesium compound of theformula (R₂)MgX or the formula Mg(R₂)₂ in which X is a chlorine orbromine atom and R₂ is an alkyl group comprising 2 to 12 carbon atoms.The reaction between the tetravalent titanium compound or compounds andthe organo magnesium compound is advantageously performed in thepresence of an alkyl halide of the formula (R₂)X in which R₂ and X havethe same definitions as above, these various compounds being employed inmolar ratios such that:

either

0.1≦TiX_(4-t) (OR₁)_(t) /(R₂)MgX≦0.33

and 1≦(R₂)X/(R₂)MgX≦2

or

0.1≦TiX_(4-t) (OR₁)_(t) /Mg(R₂)₂ ≦0.33

and 2≦(R₂)X/Mg(R₂)₂ ≦4

Another catalyst suitable for use in the process of the presentinvention comprises the product obtained by precipitating a titaniumcompound on particles obtained by reacting an organomagnesium compoundand a chlorinated organic compound, complying with the followingconditions:

the organomagnesium is either a dialkylmagnesium of the formula R₃ MgR₄,or an organomagnesium derivative of the formula R₃ MgR₄, xAl(R₅)₃, inwhich formula R₃, R₄, and R₅ are identical or different alkyl groupshaving 2 to 12 carbon atoms and x is a number comprised between 0.01 and1;

the chlorinated organic compound is an alkyl chloride of the formula R₆Cl, in which R₆ is a secondary or preferably tertiary alkyl group having3 to 12 carbon atoms;

the reaction is performed in the presence of an electron donor compoundD, which is an organic compound comprising at least one atom of oxygen,sulphur, nitrogen or phosphorus; it may be chosen from amongst a widevariety of products such as the amines, amides, phosphines, sulphoxides,sulphones or the aliphatic ether oxides.

Moreover, the various reagents used for preparing such a support may beemployed under the following conditions:

the molar ratio R₆ Cl/R₃ MgR₄ is comprised between 1.5 and 2.5, andpreferably between 1.85 and 2;

the molar ratio R₆ Cl/R₃ MgR₄, xAl(R₅)₃ is comprised between 1.5(1+3x/2) and 2.5 (1+3x/2) and preferably between 1.85 (1+3x/2) and 2(1+3x/2); the molar ratio between

the electron donor compound D and the organomagnesium (R₃ MgR₄ or R₃MgR₄, xAl(R₅)₃) is comprised between 0.1 and 1;

the reaction between the organomagnesium compound and the chlorinatedorganic compound takes place with stirring in a liquid hydrocarbon at atemperature comprised between 5° C. and 80° C.

The precipitation of the titanium compound on the solid particles may becarried out by a reduction reaction of a titanium compound of theformula TiX_(4-t) (OR₁)_(t) in which R₁, X and t have the samedefinitions as above, by means of a reducing agent chosen from amongorganomagnesium compounds of the formula R₃ MgR₄, in which R₃ and R₄have the same definitions as above, organozinc compounds of the formulaZn(R₇)_(2-y) X_(y), in which R₇ is an alkyl group having 2 to 12 carbonatoms, X is a chlorine or bromine atom and y is an integer or fractionsuch that 0≦y≦1, and organo-aluminium compounds of the formulaAl(R₈)_(3-z) X_(z), in which R₈ is an alkyl group having 2 to 12 carbonatoms, X is a chlorine or bromine atom and z is an integer or fractionsuch that 0≦z≦2;

the said reduction reaction is performed in the presence or absence ofan electron donor compound D as defined above;

the relative quantities of the various compounds (solid particles,titanium compounds, organomagnesium or organozinc or organoaluminiumcompounds, electron donor) are in molar ratios such that:

magnesium in the solid particles: titanium compound comprised between 1and 8 and preferably between 2 and 5;

organomagnesium or organozinc or organoaluminium compound: titaniumcompound less than 2 and preferably comprised between 0.5 and 1.5;

electron donor compound: titanium compound comprised between 0 and 1.

The precipitation is performed at a temperature comprised between -30°C. and 100° C. with stirring in a liquid hydrocarbon medium.

After the evaporation of the liquid hydrocarbon medium in which theyhave been prepared, the solid catalysts are ready to be used for thecopolymerisation of ethylene in the process according to the invention.

The copolymerisation is performed using as cocatalyst an organometalliccompound of a metal of groups II or III of the Periodic Table ofElements, preferably an organoaluminium or halogen-organoaluminiumcompound. The ingredients of the catalyst system must be employed inproportion such that the atomic ratio of the quantity of metal of groupsII and III in the cocatalyst to the quantity of titanium in the catalystis comprised between 0.5 and 100, preferably between 1 and 30.

The catalyst systems are employed as such or preferably after undergoinga prepolymerisation operation. This prepolymerisation operation, whichleads to prepolymer particles of a form more or less identical to thatof the initial catalyst, but of greater dimensions, consists in bringingthe catalyst and cocatalyst into contact with ethylene, possibly inadmixture with propylene and/or 1-butene and/or an alpha-olefincontaining 5 to to 8 carbon atoms. The prepolymerisation mayadvantageously be performed in two (or more) stages as described below.The commencement of the prepolymerisation reaction, or the first stageof this reaction when operated in two distinct stages, is performed insuspension in an inert liquid medium such as a liquid hydrocarbon. Thistechnique makes it possible to control the activity of the catalystsystem, especially in the initial phase of the reaction, and to avoidreaction runaways or the bursting of catalyst particles.

In the case that the prepolymerisation is carried out in two stages, thefirst prepolymerisation stage is continued until the prepolymercomprises from 0.1 to 10 g of polyethylene or copolymer of ethylene permilligram atoms of titanium. The prepolymerisation may then be continuedeither in suspension in a liquid hydrocarbon medium, or in the gasphase; generally speaking it may be continued until 10 to 300 g ofpolyethylene or copolymer of ethylene per milligram atom of titanium areobtained, whilst preserving a suitable level of activity in the catalystsystem.

The prepolymers obtained by this process occur in the form of catalyticproducts particularly suited to the copolymerisation of (a) ethylene,(b) propylene and/or 1-butene and (c) alpha-olefins comprising from 5 to8 carbon atoms, in the gas phase by means of a fluidised bed: theseprepolymers possess dimensions and a reactivity which are adapted tothis mode of copolymerisation, making it possible to obtain copolymersof a homogeneous nature in the form of non-sticking powders, which arefree in particular from liquid oligomers or copolymers with a lowmelting point, and consisting of unburst particles.

The gas-phase copolymerisation by means of a fluidised bed may beperformed according to the current techniques of polymerisation orcopolymerisation in a fluidised bed. However, the gas mixture providingfluidisstion comprises, in addition to ethylene and the comonomerspropylene and/or 1-butene and alpha-olefins comprising 5 to 8 carbonatoms, an inert gas such as nitrogen, methane or ethane and optionallyhydrogen, the latter being to provide control of the molecular weightsof the copolymers produced. The presence of an inert gas in this gaseousmixture appreciably improves the elimination of the heat of reaction andfavourably modifies the kinetics of copolymerisation. The speed offluidisation in the fluidised bed reactor is preferably sufficientlyhigh to assure homogenisation of the fluidised bed and to eliminateeffectively the heat evolved by the copolymerisation without havingrecourse to other means of homogenisation, especially mechanical orinvolving the use of a readily volatile liquid. The speed offluidisation is preferably between 6 and 10 times the minimum speed offluidisation, that is to say generally between about 40 and 80 cm/sec.In passing throught the fluidised bed, only a part of the ethylene andthe comonomers is polymerised in contact with the particles of copolymerin course of growth. The gaseous mixture containing the unreactedfraction of ethylene and comonomers leaves the fluidised bed and passesthrough cooling system intended to eliminate the heat produced duringthe reaction before being recycled into the fluidised bed by means of acompressor.

The copolymerisation is performed at a temperature comprised between 50°C. and 100° C., preferably between 70° C. and 90° C. under a totalpressure generally between 0.5 and 4 MPa. The copolymerisation isadvantageously stopped when the copolymer contains per gram less than5×10⁻⁴ milligram atoms of titanium and preferably less than 4×10⁻⁴milligram atoms of titanium.

As comonomer (c), preference is given to 4-methyl-1-pentene, 1hexene or1-octene.

The invention also concerns copolymers of (a) ethylene, (b) propyleneand/or 1-butene and (c) alpha-olefins comprising 5 to 8 carbon atoms,these copolymers:

having a density comprised between 0.900 and 0.935,

containing approximately 4 to 15% by weight of the total of the unitsderived from comonomers (b) and (c), and

containing units derived from comonomers (b) and (c) in a quantity suchthat the ratio by weight of the quantity of comonomer (c) to thequantity of comonomer (b) is comprised between 0.1 and 1.5, preferablybetween 0.25 and 1.

It has surprisingly been found that by a synergic effect, the density ofthe ethylene copolymers containing the two comonomers (b) and (c) isreduced as compared with that of a copolymer containing only one ofthese two comonomers, in a quantity by weight equivalent to that of thetwo comonomers (b) and (c) involved simultaneously. The advantageousresult is that a copolymer of a given density prepared according to theinvention has a content by weight of comonomers (b) and (c) lower thanthat of a copolymer of identical density which contains only one ofthese two comonomers. The synergic effect observed in the simultaneoususe of the two comonomers (b) and (c) is particularly remarkable whenthe copolymer contains these two comonomers in a quantity such that theratio by weight of the quantity of comonomer (c) to the quantity ofcomonomer (b) is comprised between 0.1 and 1.5, preferably between 0.25and 1.

Furthermore, it is found that the copolymers which have a density equalto or greater than 0.918 have a content of copolymers soluble inn-hexane at 50° C. less than or equal to 2.0% by weight, a content whichis substantially lower than that of copolymers of ethylene and comonomer(c) of identical density. It has also been found that in the fraction ofthe copolymers which is soluble in boiling n-hexane, the total contentin comonomers (b) and (c) does not exceed the total content in comonmers(b) and (c) of the fraction of the copolymers which is insoluble inboiling n-heptane by more than 15%. This characteristic is shown indetail in the Examples and in Tables 1 and 3.

Due to these characteristics, the copolymers of the invention can bemanufactured by fluidised bed copolymerisation without difficulty,thanks particularly to the relatively low partial pressure of thecomonomers (c) and to the non-sticky character of the copolymerarticles. The non-sticky character of the particles also allows an easyhandling of the copolymers.

The density of the copolymers is not limited to values equal to orgreater than 0.918, it being possible to obtain without difficulty andwith high yield copolymer of a lower density.

Moreover, the copolymer powders prepared according to the inventionconsist substantially of unburst particles; they are easy to handle andhave a relatively high bulk density comprised between 0.30 and 0.45g/cm³ and which in particular is independent of the yield of copolymerby the reaction in relation to the catalyst. Furthermore, these powderscomprise less than 350 ppm, preferably less than 150 ppm of inorganicresidues which are totally free from mineral compounds based onrefractory oxides of the alumina and/or silica type.

By differential scanning calorimetric analysis, after stoving at 200°C., cooling at a rate of 16° C. per minute, and heating at a rate of 16°C. per minute, the copolymers prepared according to the invention show asingle melting point at a temperature comprised between 116° and 128°C., the melting point diagram characteristically showing a single peakat this temperature, which corresponds to a special distribution of thedimensions of the crystallites. The fusion enthalpy of these copolymerscorresponds to a crystallinity comprised between about 25 and 50%.

The copolymers of the invention have a flow parameter n comprisedbetween 20 and 40, calculated by the ratio of the melt index (MI₂₁.6)measured under 21.6 kg to the melt index (MI₂.16) measured under 2.16kg.

The copolymers of the invention also possess a relatively narrowmolecular weight distribution, such that the ratio of the weight averagemolecular weight, Mw, to the number average molecular weight, Mn,determined by gel permeation chromatography, is comprised between 3 and5.5 and more especially comprised between 4 and 5.

These copolymers are also characterised by a very low level of ethylenicunsaturation of the vinyl, vinylene and vinylidene type, less than 0.2ethyenic double bond per 1000 atoms of carbon, which confers on thesecopolymers an excellent stability. Moreover, according to the carbon 13nuclear magnetic resonance analysis (NMR) the molecular structure of thecopolymers of the invention is such that the units derived fromcomonomers (b) and (c) are distributed randomly along the copolymerchain, at least 95% of these units being isolated from each other, andseparated by more than one unit derived from ethylene.

The structure of the copolymers according to the invention ischaracterised in addition by a very low level of long chain branching(g*) which is expressed by a value g*=(η)/(η₁) 0.90, (η) being theintrinsic viscosity of a given copolymer and (η₁) being the intrinsicviscosity of a linear polyethylene having the same weight averagemolecular weight as that of the said copolymer.

These copolymers, whose fluidity index measured under 2.16 kg may varybetween 0.1 and 30 g per 10 minutes, find numerous applications in theproduction of finished objects by injection moulding or rotationalmoulding techniques, or extrusion forming or blowing extrusiontechniques, and particularly applications which are of interest in theproduction of films with a high mechanical strength.

The following non-restrictive Examples illustrate the invention.

EXAMPLE 1 Preparation of the catalyst

Into a 1-liter glass flask, provided with a stirrer system and a heatingand cooling device, there are introduced under an atmosphere of nitrogenat 20° C., 500 ml of n-heptane, 8.8 g of magnesium in powder form and1.2 g of iodine successively. With stirring, the reaction mixture isheated to 80° C. and there are rapidly introduced 9.1 g of titaniumtetrachloride and 13.7 g of tetrapropyltitanate, then slowly over 4hours 74.5 g of n-butyl chloride. At the end of this period the reactionmixture thus obtained is maintained for 2 hours with stirring and at 80°C., then it is cooled to ambient temperature (20° C.). The precipitateobtained is then washed 3 times with n-hexane to give the solid catalyst(A) ready for use. Analysis of the catalyst (A) obtained shows that itcontains per gram atom of total titanium:

0.9 gram atom of trivalent titanium,

0.1 gram atom of tetravelent titanium,

3.7 gram atoms of magnesium and

7.7 gram atoms of chlorine

and that the composition of the catalyst (A) corresponds to the generalformula:

    Mg.sub.3.3 Ti(OC.sub.3 H.sub.7).sub.2 (C.sub.4 H.sub.9).sub.0.7 Cl.sub.7.7

Preparation of the prepolymer

Into a 5-liter stainless steel reactor, provided with a stirrer systemrotating at 750 revolutions per minute, there are introduced undernitrogen 3 liters of n-hexane which is heated to 70° C., 25 millilitersof a molar solution of tri-n-octyl aluminium (TnOA) to n-hexane and aquantity of catalyst (A) prepared before hand containing 12.5 milligramatoms of titanium. The reactor is then closed and there are introducedhydrogen up to a pressure of 0.05 MPa and ethylene at a throughput of160 g/hr for 3 hours. The prepolymer obtained (B) is then dried in arotating evaporator under vaccuum and preserved under nitrogen. Itcontains 0.026 milligram atoms of titanium per gram.

Copolymerisation

Into a fluidised bed reactor with a diameter of 90 cm, operating with arising gas mixture propelled at a speed of 45 cm/sec and under partialpressures (pp) of:

pp hydrogen: 0.051 MPa,

pp ethylene: 0.46 MPa,

pp 1-butene: 0.11 MPa,

pp 4-methyl-1-pentene: 0.028 MPa and

pp nitrogen: 0.96 MPa,

at a temperature of 80° C., there are introduced 350 kg of an anhydrouspolyethylene powder as the charge powder, then in sequence 96 g ofprepolymer (B) every 5 minutes. By sequenced withdrawal, 90 kg per hourof a copolymer powder collected, whilst maintaining constant the heightof the fluidised bed. After 12 hours of copolymerisation under theseconditions, the charge powder is practically completely eliminated and acopolymer powder (C) is obtained having the following characteristics:

density of the copolymer: 0.918;

content of units derived from the comonomer (b) (1-butene): 5.5% byweight;

content of units derived from comonomer (b) (4-methyl-1-pentene): 2.5%by weight;

melt index (MI₂.16): 1.0 g/10 minutes;

titanium content: 3.3×10⁻⁴ milligram atoms of titanium per gram;

bulk density: 0.37 g/cm³ ;

ethylenic unsaturation level: 0.15 ethylenic double bond per 1000 carbonatoms;

melting point: 123° C.;

fusion enthalpy (Delta Hf): 100 J/g;

long branching level (g*): 0.93;

molecular weight distribution (Mw/Mn): 4.0;

content of copolymers soluble in n-hexane at 50° C.: 1.7% by weight.

The values of these characteristics are summarised in Table 1.

On an RCB granulator, granulates are prepared from the copolymer powder(C) by mixing:

98.8% by weight of this polymer,

0.1% by weight of calcium stearate,

0.02% by weight of a phenolic compound sold by Ciba-Geigy under thetrade name "Irganox 1076"®, and

0.08% by weight of an organophosphorus compound sold by Ciba-Geigy underthe trade name "Irgafos 168"®.

These pellets are then converted into a film with a thickness of 35microns by means of a "Semivex ESY 45" extruder under the followingconditions:

air gap on the die: 2.0 mm;

swelling index: 2;

temperature: 225° C.;

throughput: 15 kg/hr

The mechanical and optical properties of these films are determined bymeasuring the tearing and perforation strengths of the said films, andalso their transparency and gloss. Results of these measurements aregiven in Table 2.

EXAMPLE 2 Copolymerisation

This is identical to that of Example 1, except for the fact that insteadof using a gas mixture containing 0.051 MPa of hydrogen, 0.46 MPa ofethylene, 0.11 MPa of 1-butene, 0.028 MPa of 4-methyl-1-pentene and 0.96MPa of nitrogen, a gas mixture is used the various ingredients of whichhave the following partial pressures (pp):

pp hydrogen: 0.05 MPa,

pp ethylene: 0.50 MPa,

pp 1-butene: 0.096 MPa,

pp 4-methyl-1-pentene: 0.055 MPa and

pp nitrogen: 0.90 MPa.

After 12 hours of copolymerisation, a copolymer powder (D) is obtainedthe characteristics of which are given in Table 1.

This copolymer powder is converted into pellets, then into films, underconditions identical to those of Example 1, except for the fact thatinstead of using the powder of copolymer (C), the powder of copolymer(D) is employed. The films this obtained have very good mechanical andoptical properties, as is shown by the results of the measurements,given in Table 2.

EXAMPLE 3 (comparative) Copolymerisation

This is identical to that of Example 1, except for the fact that insteadof using a gas mixture containing 0.051 MPa of hydrogen, 0.46 MPa ofethylene, 0.11 MPa of 1-butene, 0.028 MPa of 4-methyl-1-pentene and 0.96MPa of nitrogen, a gas mixture is used the various ingredients of whichhave the following pressures (pp).

pp hydrogen: 0.08 MPa,

pp ethylene: 0.46 MPa

pp 1-butene: 0.16 MPa

pp nitrogen: 0.90 MPa.

After 12 hours of copolymerisation, a copolymer powder (E) is obtainedthe characteristics of which are given in Table 1. An examination ofthis Table shows in particular that, compared to copolymers (C) and (D)containing ethylene, 1-butene and 4-methyl-1-pentene, copolymer (E)which contains similar total quantities of ethylene and 1-butene,exhibit mechanical and optical properties substantially lower than thoseof the films obtained from the powders of copolymers (C) and (D), as isshown by the results of measurements given in Table 2.

This copolymer powder is converted into pellets, then into films, underconditions identical to those of Example 1, except for the fact thatinstead using the powder of copolymer (C), the powder of copolymer (E)is employed. The films thus obtained have considerably poorer mechanicaland optical properties than those of the films obtained from the powdersof copolymers (C) and (D), as is shown by the results of measurementsgiven in Table 2.

EXAMPLE 4 (comparative) Copolymerisation

This is identical to that of Example 1, except for the fact that insteadof using a gas mixture containing 0.051 MPa of hydrogen, 0.46 MPa ofethylene, 0.11 MPa of 1-butene, 0.028 MPa of 4-methyl-1-pentene and 0.96MPa of nitrogen, a gas mixture is employed the various ingredients ofwhich have the following partial pressures (pp):

pp hydrogen: 0.11 MPa,

pp ethylene: 0.338 MPa

pp 4-methyl-1-pentene: 0.074 MPa, and

pp nitrogen: 1.078 MPa.

After 12 hours of copolymerisation, a copolymer powder (F) is obtainedconsisting of relatively sticky particles which are difficult to handle.The characteristics of this powder are given in Table 1. An examinationof this table shows in particular that, as compared with copolymers (C)and (D) of ethylene, 1-butene and 4-methyl-1-pentene, the copolymer (F)of ethylene and 4-methyl-1-pentene has for an equivalent density, asubstantially higher content by weight of comonomer, and also adistinctly larger content of copolymers soluble in n-hexane at 50° C.

This copolymer powder is converted into pellets, then into films underconditions identical to those of Example 1, except for the fact thatinstead of using the powder of copolymer (C), the powder of copolymer(F) is employed. The films thus obtained have mechanical propertiesslightly inferior to those of the films obtained from the powder ofcopolymers (C) and (D), as is shown by the results of the measurementsgiven in Table 2.

EXAMPLE 5 (comparative) Copolymerisation

This is identical to that of Example 1, except for the fact that insteadof using a gas mixture containing 0.051 MPa of hydrogen, 0.46 MPa ofethylene, 0.11 MPa of 1-butene, 0.028 MPa of 4-methyl-1-pentene and 0.96MPa of nitrogen, a gas mixture is employed the various ingredients ofwhich have the following partial pressures (pp):

pp hydrogen: 0.04 MPa,

pp ethylene: 0.45 MPa,

pp 1-butene: 0.07 MPa,

pp 4-methyl-1-pentene: 0.08 MPa, and

pp nitrogen: 0.96 MPa.

After 12 hours of copolymerisation a copolymer powder (G) is obtained,which consists of particles having a relatively sticky character andwhich are difficult to handle. The characteristics of this powder aregiven in Table 1. This Table shows, in particular, that compared tocopolymers (C) and (D), copolymer (G) has a content in4-methyl-1-pentene substantially higher that its content in 1-butene.The fraction of copolymer (G) which is soluble in n-hexane at 50° C. isalso substantially higher than for copolymers (C) and (D).

This copolymer powder is converted into pellets, then into films, underconditions identical to those for Example 1, except for the fact thatinstead of using the copolymer (C), copolymer powder (G) is employed.The films thus obtained have mechanical and optical properties, whichare not as good as those measured on the films obtained from the powdersof copolymers (C) and (D), as is shown in Table 2.

EXAMPLE 6 Copolymerisation

This is identical to that of Example 1, except for the fact that insteadof using a gas mixture containing 0.051 MPa of hydrogen, 0.46 MPa ofethylene, 0.11 MPa of 1-butene, 0.028 MPa of 4-methyl-1-pentene and 0.96MPa of nitrogen, a gas mixture is employed the various ingredients ofwhich have the following partial pressures (pp):

pp hydrogen: 0.045 MPa,

pp ethylene: 0.465 MPa,

pp 1-butene: 0.12 MPa,

pp 4-methyl-1-pentene: 0.07 MPa, and

pp nitrogen: 0.90 MPa.

After 12 hours of copolymerisation, a copolymer powder (H) is obtainedthe characteristics of which are given in Table 3. This copolymer,despite a comparatively low density, occurs in the form of a powderconsisting of non-sticky particles, having a low content of copolymerssoluble in n-hexane at 50° C.

This copolymer powder is converted into pellets, then into film underconditions identical to those of Example 1, except for the fact thatinstead of using the copolymer powder (C), the copolymer powder (H) isutilised. The films thus obtained have excellent mechanical and opticalproperties as is shown by the results of the measurements given in Table4.

EXAMPLE 7 Copolymerisation

This is identical to that of Example 1, except for the fact that insteadof using a gas mixture containing 0.051 MPa of hydrogen, 0.46 MPa ofethylene, 0.11 MPa of 1-butene, 0.028 MPa of 4-methyl-1-pentene and 0.96MPa of nitrogen, a gas mixture is employed the various ingredients ofwhich have the following partial pressures (pp):

pp hydrogen: 0.09 MPa,

pp ethylene: 0.44 MPa,

pp 1-butene: 0.013 MPa,

pp 4-methyl-1-pentene: 0.04 MPa, and

pp nitrogen: 0.90 MPa.

After 12 hours of copolymerisation a copolymer powder (I) is obtained,the characteristics of which are given in Table 3. An examination ofthis Table shows in particular that the copolymer (I) has a particularlylow density, despite a fairly low content by weight of comonomers.

This comonomer powder is converted into pellets, then into films, underconditions identical to those of Example 1, except for the fact thatinstead of using the copolymer (C) copolymer powder (I) is employed. Thefilms thus obtained have particularly remarkable mechanical and opticalproperties, bearing in mind the low density of the copolymer, as shownby the results of the measurements given in Table 4.

EXAMPLE 8 Copolymerisation

This is identical to that of Example 1, except for the fact that insteadof using a gas mixture containing 0.051 MPa of hydrogen, 0.46 MPa ofethylene, 0.11 MPa of 1-butene, 0.028 MPa of 4-methyl-1-pentene and 0.96MPa of nitrogen, a gas mixture is employed the various ingredients ofwhich have the following partial pressures (pp):

pp hydrogen: 0.06 MPa

pp ethylene: 0.49 MPa

pp 1-butene: 0.08 MPa

pp 4-methyl-1-pentene: 0.05 MPa and,

pp nitrogen: 0.92 MPa.

After 12 hours of copolymerisation, a copolymer powder (J) is obtainedthe characteristics of which are given in Table 3.

This copolymer powder is converted into pellets, then into films, underconditions identical to those of Example 1, except for the fact thatinstead of using copolymer powder (C), copolymer powder (J) is employed.The films thus obtained have good mechanical and optical properties, asis shown by the results of the measurements given in Table 4.

EXAMPLE 9 (comparative) Copolymerisation

This is identical to that of Example 1, except for the fact that insteadof using a gas mixture containing 0.051 MPa of hydrogen, 0.46 MPa ofethylene, 0.11 MPa of 1-butene, 0.028 MPa of 4-methyl-1-pentene and 0.96of nitrogen, a gas mixture is employed the various ingredients of whichhave the following partial pressures (pp):

pp hydrogen: 0.10 MPa

pp ethylene: 0.45 MPa

pp 1-butene: 0.14 MPa

pp nitrogen: 0.91 MPa

After 12 hours of copolymerisation a copolymer powder (K) is obtained,the characteristics of which are given in Table 3. An example of thisTable shows in particular that, as compared with copolymer (J) ofethylene, 1-butene and 4-methyl-1-pentene, copolymer (K) of ethylene and1-butene has, for a similar content by weight of comonomer, an identicaldensity.

This copolymer powder is converted into pellets, then into films, underconditions identical to those of Example 1, except for the fact thatinstead of using copolymer powder (C), copolymer powder (K) is employed.The films thus obtained have relatively lower mechanical and opticalproperties than those of the films obtained from copolymer powder (K),as is shown by the results of the measurements given in Table 4.

MEASUREMENT OF THE MOLECULAR WEIGHT DISTRIBUTION

The molecular weight distribution of the copolymer is calculatedaccording to the ratio of the weight average molecular weight, Mw, tothe number average molecular weight, Mn, of the copolymer, from adistribution curve for the molecular weights obtained by means of a gelpermeation chromoatrograph of the "DuPont" Type 860 make (hightemperature size exclusion chromatrograph), provided with a pump of"DuPont" type 870, the operating conditions being as follows:

solvent: trichloro-1,2,4-benzene

throughput of solvent: 0.8 ml/minute

three columns of the DuPont type with a "Zorbax" packing, the particleof which having a of size 6 microns and a porosity of 60 521 , 1000 Åand 4000 Å respectively

temperaure: 150° C.

concentration of sample: 0.15% by weight

injection volume: 300 ml

detection by infrared, at a wave length of 3.425 microns, by means of acell 1 mm thick

standardisation by means of a high density polyethylene sold by BPChimie SA under the trade name "Natene 6055"®: Mw=70 000 and Mw:Mn=3.8

METHOD FOR DETERMINING THE LEVEL OF LONG BRANCHING, G*

In the formula g*=(η)/(η)₁, the intrinsic viscosity (η) of the copolymeris measured in trichlorobenzene at 135° C. For its part the intrinsicviscosity (η)₁ of the linear polyethylene having the same weight averagemolecular weight, Mw, as the said copolymer, is calculated according tothe following Mark-Houwink equation: (η)₁ equals 6.02×10⁻⁴ ×(Mw)0.69;the weight average molecular weight, Mw of the copolymer is determinedby gel permeation chromatography, the fractionation columns beingstandardised by means of linear polyethylene.

MEASUREMENT OF THE MELT INDICES (MI₂.16) AND (MI₂₁.6)

The melt index (MI₂.16 is measured under a load of 2.16 kg at 190° C. bythe ASTM D-1238 method condition (E).

The melt index (MI₂₁.6) is measured under a load of 21.6 kg at 190° C.,by the ASTM D-1238 method, condition F.

MEASUREMENT OF THE LIQUID OLIGOMER CONTENT

The liquid oligomer content is the percentage of the weight of thefraction of the copolymers which is dissolved in ethyl ether at 20° C.after 24 hours. The lower limit of the measurement is of 0.05% byweight. The nature of the liquid oligomers is determined by gaschromatography; they generally correspond to hydrocarbons comprisingless than 25 carbon atoms.

MEASUREMENT OF THE CONTENT OF COPOLYMERS SOLUBLE IN N-HEXANE AT 50° C.

This measurement corresponds to the method used for FDA standards in thecase of polyethylene films for foods stuff packaging. According to thismethod, a sample of a film having a thickness of 100 micron and theshape of a square of 25×25 mm is dipped in 300 ml of n-hexane at 50° C.and maintained under agitation during 2 hours. The film is then driedand weighed. The content of copolymers soluble in expressed according tothe difference in the weights of the film before and after thetreatment.

MEASUREMENT OF THE CONTENT OF COPOLYMERS SOLUBLE IN BOILING N-HEXANE

This measurement is made in the same manner as for measuring the contentof copolymers soluble in n-hexane at 50° C., except for the n-hexane ismaintained at its boiling temperature.

MEASUREMENT OF THE CONTENT OF COPOLYMERS SOLUBLE IN BOILING N-HEPTANE

This measurement is made in the same manner as for measuring the contentof copolymers soluble in boiling n-hexane, except that n-heptane is usedinstead of n-hexane.

MEASUREMENTS ON FILM

tearing strengths (in machine direction and transverse direction)measured according to standard ASTM D-1922;

perforation strength, measured according to standard ASTM D-781;

perforation strength ("Dart test") measured according to standard ASTMD-1709;

Transparency (or "Haze") measured according to standard ASTM D-1003;

brillancy ("Gloss") measured according to standard ASTM D-2457.

                                      TABLE 1                                     __________________________________________________________________________                    Example                                                                                   3     4     5                                                                 (com- (com- (com-                                                 1     2     parative)                                                                           parative)                                                                           parative)                             __________________________________________________________________________    Density (at 20° C.)                                                                    0.918 0.918 0.919 0.918 0.918                                 Content (% by                                                                 weight of units                                                                           (b) 5.5   4.0   7.8   --    3                                     derived from the                                                              comonomers  (c) 2.5   3.8   --    11.0  6.9                                   MI.sub.2·16 (g/10 mins)                                                              1.0   0.9   1.0   1.0   1                                     Content of titanium                                                           (mg. at. Ti/g)  3.3 × 10.sup.-4                                                               3.3 × 10.sup.-4                                                               3.8 × 10.sup.-4                                                               4.2 × 10.sup.-4                                                               4.2 × 10.sup.-4                 Bulk density (g/cm.sup.3)                                                                     0.37  0.36  0.37  0.31  0.32                                  Level of ethylenic                                                            unsaturation/1000                                                             carbon atoms    0.15  0.17  0.14  0.18  0.17                                  Melting point (°C.)                                                                    123   123   123   124   124                                   Fusion Enthalpy,                                                              delta Hf (J/g)  100   93.3  96.3  103   101                                   Long branching                                                                level, g*       0.93  0.95  0.96  0.91  0.92                                  Mw/Mn           4.0   4.0   4.4   4.3   3.0                                   Flow parameter, n                                                                             25    28    34    32    33                                    Content of copolymers                                                         soluble in n-hexane                                                           at 50° C. (% by weight)                                                                1.7   1.8   1.5   4.5   4.4                                   Weight  Soluble                                                                             b.sub.1                                                                         9     7.9   17.0  --    6.2                                   content (%)                                                                           in boiling                                                            of units                                                                              n-hexane                                                                            c.sub.1                                                                         8.4   8.9   --    27.0  19.0                                  derived from                                                                  comonomers (b)                                                                        Soluble in                                                                          b.sub.2                                                                         7.2   7.2   10.9  --    4.8                                   and (c), in                                                                           boiling                                                               the fractions                                                                         n-heptane                                                                           c.sub.2                                                                         5.5   5.8   --    21.0  12.0                                  which are:                                                                            Insoluble                                                                           b.sub.3                                                                         3.7   3.6   3.9   --    2.5                                           in boiling                                                                    n-heptane                                                                           c.sub.3                                                                         2.0   3.4   --    5.6   4.3                                   Total   B = b.sub.1 -b.sub.2                                                                  5.3   4.3   13.1  --    3.7                                   difference                                                                            C = c.sub.1 -c.sub.3                                                                  6.4   5.5   --    21.4  14.7                                  in monomer                                                                    content B + C   11.7  9.8   13.1  21.4  18.4                                  __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________                   Example                                                                             3    4    5                                                                   (compar-                                                                           (compar-                                                                           (compar-                                                      1  2  ative)                                                                             ative)                                                                             ative)                                         __________________________________________________________________________    Copolymer      C  D  E    F    G                                              Density (at 20° C.)                                                                   0.918                                                                            0.918                                                                            0.919                                                                              0.918                                                                              0.918                                          Content (% by                                                                 weight of units                                                                        (b)   5.5                                                                              4.0                                                                              7.8  --   9                                              derived from                                                                  the comonomers                                                                         (c)   2.5                                                                              3.8                                                                              --   11.0 6.9                                            Tearing strength                                                                       Machine                                                              according to                                                                           direction                                                                           3.43                                                                             3.72                                                                             1.27 2.45 2.07                                           standard ASTM                                                                 D 1922 (N)                                                                             Transverse                                                                    direction                                                                           4.90                                                                             6.47                                                                             3.92 >10  10                                             Perforation strength                                                          according to standard                                                                        21 20 14   16   15                                             ASTM D 781 (dJ)                                                               Perforation strength                                                          according to   1.52                                                                             1.42                                                                             1.07 1.37 1.25                                           standard ASTM D 1709                                                          ("Dart Test") (N)                                                             Optical Properties                                                                     Haze  17 14 16   17   17                                                      Gloss 47 46 40   45   43                                             __________________________________________________________________________

                                      TABLE 3                                     __________________________________________________________________________                    Example                                                                                         9                                                                             (com-                                                       6     7     8     parative)                                   __________________________________________________________________________    Density (at 20 ° C.)                                                                   0.914 0.915 0.922 0.922                                       Content (% by                                                                 weight of units                                                                           (b) 4.8   6.5   3.3   6.8                                         derived from the                                                              comonomers  (c) 6.2   3.5   3.6   --                                          MI.sub.2·16 (g/10 mins)                                                              1.7   0.8   1.0   0.9                                         Content of titanium                                                           (mg. at. Ti/g)  3.0 × 10.sup.-4                                                               3.5 × 10.sup.-4                                                               3.3 × 10.sup.-4                                                               3.7 × 10.sup.-4                       Bulk Density (g/cm.sup.3)                                                                     0.32  0.33  0.38  0.37                                        Level of ethylenic                                                            unsaturation/1000                                                                             0.16  0.15  0.17  0.15                                        carbon atoms                                                                  Melting point (° C.)                                                                   123   123   123   124                                         Fusion Enthalpy,                                                              delta Hf (J/g)  90.4  94.2  103.0 109.7                                       Long branching  0.92  0.95  0.94  0.96                                        level, g*                                                                     Mw/Mn           5.0   4.7   4.1   4.4                                         Flow parameter, n                                                                             35    34    28    32                                          Content of copolymers                                                         soluble in n-hexane                                                                           2.5   2.4   1.4   1.2                                         at 50° C.(% by weight                                                  Weight  Soluble                                                                             b.sub.1                                                                         8.4   10.3   8.1  15.2                                        content (%)                                                                           in boiling                                                            of units                                                                              n-hexane                                                                            c.sub.1                                                                         13.1  8.1   8.8   --                                          derived from                                                                  comonomers (b)                                                                        Soluble in                                                                          b.sub.2                                                                         7.4   8.4   6.0   10.1                                        and (c), in                                                                           boiling                                                               the fractions                                                                         n-heptane                                                                           c.sub.2                                                                         8.1   5.3   5.5   --                                          which are:                                                                            Insoluble                                                                           b.sub.3                                                                         3.9   3.8   2.9   3.2                                                 in boiling                                                                    n-heptane                                                                           c.sub.3                                                                         3.8   3.3   3.3   --                                          Total   B = b.sub.1 -b.sub.2                                                                  4.5   6.5   5.2   12.0                                        difference                                                                            C = c.sub.1 -c.sub.3                                                                  9.4   4.8   5.5   --                                          in monomer                                                                    content B + C   13.9  11.3  10.7  12.0                                        __________________________________________________________________________

                  TABLE 4                                                         ______________________________________                                                         Example                                                                                          9                                                                             (com-                                                      6    7      8      parative)                                 ______________________________________                                        Copolymer          H      I      J    K                                       Density (at 20° C.)                                                                       0.914  0.915  0.922                                                                              0.922                                   Content (% by weight)                                                         of units derived                                                                           (b)       4.8    6.5  3.3  6.8                                   from the comonomers                                                                        (c)       6.2    3.5  3.6  --                                    Tearing strength                                                                           Machine                                                          according to standard                                                                      direction >5     4.90 1.56 0.88                                  ASTM D 1922 (N)                                                                            Transverse                                                                    direction >5     7.15 5.39 3.92                                  Perforation strength according                                                to standard ASTM D 781 (dJ)                                                                      24     23     15   11                                      Perforation strength according                                                to standard ASTM D 1709                                                                          2.05   2.01   1.17 0.58                                    ("Dart Test") (N)                                                             Optical Properties                                                                         Haze      13     16   19   20                                                 Gloss     50     47   42   40                                    ______________________________________                                    

We claim:
 1. A gas fluidised bed process for the production ofcopolymers having a density comprised between 0.900 and 0.935, thisprocess being characterised in that it comprises a copolymerisation of(a) ethylene, (b) propylene and/or 1-butene, and (c) alpha-olefinscomprising 5 to 8 carbon atoms in the gaseous state in admixture with aninert gas and optionally hydrogen, the gas mixture circulating frombottom to top through the fluidised bed of the copolymer in course offormation, the copolymerisation being effected at a temperaturecomprised between 50° C. and 100° C. under a total pressure comprisedbetween 0.5 to 4 MPa with a speed of fluidisation from 40 to 80 cm/secin the presence of a catalyst system comprising:a solid catalystprepared by reacting at between -20° C. and 150° C. magnesium metal withan alkyl halide R₂ X and one or more tetravalent titanium compoundshaving the formula TiX_(4-t) (OR₁ 1)_(t), wherein R₁ is an alkyl groupcontaining 2 to 8 carbon atoms, R₂ is an alkyl group containing 2 to 12carbon atoms, X is chlorine or bromine and t is an integer or fractionfrom 0 to 3; in molar ratios such that:0.1≦TiX_(4-t) (OR₁)_(t) /Mg≦0.33and 0.5≦(R₂ X/Mg≦8 and, a cocatalyst consisting of at least oneorganoaluminium or halogen-organoaluminium compound in a quantity suchthat the atomic ratio of the quantity of aluminium to the quantity oftitanium is comprised between 1 and 30, the partial pressure (pp) of thevarious ingredients of the gas mixture being such that:0.05≦pp comonomer(b)/pp ethylene ≦0.4 0.05≦pp comonomer (c)/pp ethylene ≦0.2 0≦pphydrogen/pp ethylene ≦0.5 0.2≦pp inert gas/total pressure ≦0.8, and 0.01Mpa≦pp comonomer (c) ≦0.1 MPa and such that the ratio by weight of thequantity of comonomer (c) to the quantity of comonomer (b) fixed in thecopolymer is comprised between 0.1 and 1.5, comonomer (b) beingpropylene or 1-butene or a mixture of these two olefins, comonomer (c)being an alpha-olefin comprising 5 to 8 carbon atoms or a mixture ofthese alpha olefins, further characterised in that before performing insaid copolymerisation, the catalyst system is subjected toprepolymerisation during which the catalyst and cocatalyst are broughtinto contact with ethylene, so as to obtain from 0.1 to 300 g ofpolyethylene or copolymer of ethylene per milligramme atom of titanium.2. Process in accordance with claim 1, characterised in that the solidcatalyst is obtained by reacting at between 60° C. and 90° C., magnesiummetal with an alkyl halide R₂ X and one or more tetravalent titaniumcompounds having the formula TiX_(4-t) (OR₁)_(t) wherein t is an integeror a fraction from 0 to
 3. 3. Process in accordance with claim 1,characterised in that the commencement of prepolymerisation is carriedout in suspension in a liquid hydrocarbon medium.
 4. Process inaccordance with claim 1, characterised in that the prepolymerisation iscarried out in two stages, the first stage being performed in suspensionin a liquid hydrocarbon medium, so as to obtain from 0.1 to 10 g ofpolymer or copolymer per milligramme atom of titanium, the second stagebeing performed either in suspension in a liquid hydrocarbon medium orin the gas phase.
 5. A novel copolymer of (a) ethylene, comonomer (b)being propylene or 1-butene or a mixture of these two olefins, andcomonomer (c) being an alpha olefin comprising 5 to 8 carbon atoms or amixture of these alpha olefins, said copolymer being produced by a gasfluidised bed process comprising copolymerising said comonomers (a), (b)and (c), in the gaseous state in admixture with an inert gas andoptionally hydrogen, the gas mixture circulating from bottom to topthrough the fluidised bed of the copolymer in course of formation, thecopolymerisation being effected at a temperature comprised between 50°C. and 100° C. under a total pressure comprised between 0.5 to 4 MPawith a speed of fluidisation from 40 to 80 cm/sec in the presence of acatalyst system comprising:a solid catalyst prepared by reacting atbetween -20° C. and 150° C. magnesium metal with an alkyl halide R₂ andone or more tetravalent titanium compounds having the formula TiX_(4-t)(OR₁)_(t1), wherein R₁ is an alkyl group containing 2 to 8 carbon atoms,R₂ is an alkyl group containing 2 to 12 carbon atoms, X is chlorine orbromine and t is an integer or fraction from 0 to 3, in molar ratiossuch that:0.1≦TiX_(4-t) (OR₁)_(t) /Mg≦0.33 and 0.5≦(R₂)X/Mg≦8 and acocatalyst consisting of at least one organoaluminium compound in aquantity such that the atomic ratio of the quantity of aluminium to thequantity of titanium is comprised between 1 and 30, the partial pressure(pp) of the various ingredients of the gas mixture being suchthat0.05≦pp comonomer (b)/pp ethylene ≦0.4 0.05≦pp comonomer (c)/ppethylene ≦0.2 0≦pp hydrogen/pp ethylene ≦0.5 0.2≦pp inert gas/totalpressure ≦0.8, and 0.01 MPa≦pp comonomer (c)/0.1 MPa comonomer (b) andcomonomer (c) being defined above, further characterised in that beforeperforming the said copolymerisation, the catalyst system is subjectedto prepolymerisation during which the catalyst and cocatalyst arebrought into contact with ethylene, so as to obtain from 0.1 to 300 g ofpolymer of copolymer per milligramme atom of titanium, said copolymerbeing characterised in that it has:(i) a density comprised between 0.900and 0.935, (ii) a total content of units derived from comonomers (b) and(c) comprised between 4 and 15% by weight (iii) contents of unitsderived from comonomers (b) and (c) such that the ratio by weight of thequantity of comonomer (c) to the quantity of comonomer (b) is comprisedbetween 0.1 and 1.5, (iv) a melt index (MI₂.16) measured under 2.16 kg,comprised between 0.1 and 30 g/10 minutes, (v) a flow parametercomprised between 20 and 40, calculated by the ratio of the melt index(MI₂₁.6) measured under 21.6 kg to the melt index (MI₂.16) measuredunder 2.16 kg, (vi) a content of copolymers soluble in n-hexane at 50°C. less than or equal to 2.0% by weight, when the copolymer has adensity equal to or greater than 0.918, (vii) a total content ofcomonomers (b) and (c) present in the fraction of the copolymers whichis soluble in boiling n-hexane which does not exceed by more than 15%the total content in comonomers (b) and (c) present in the fraction ofthe copolymer which is insoluble in boiling n-heptane, (viii) anethylenic unsaturation level of the vinyl, vinylene, and vinylidene typeof less than 0.2 ethylenic double bond per 1000 carbon atoms, (ix) asingle melting point comprised between 116° and 128° C., determined byscanning differential calorimetric analysis after stoving at 200° C.,cooling at a speed of 16° per minute and heating at a speed of 16° C.per minute, (x) a structure with a low level of long chain branchings,such that g*≦0.90, g*=(η):(η₁), (η) being the intrinsic viscosity of thecopolymer and (η₁) being the intrinsic viscosity of a linearpolyethylene having the same weight average molecular weight as that ofthe said copolymer, (k) the copolymer contains per gram less than 5×10⁻⁴milligram atoms or titanium, (l) the bulk density is between 0.30 and0.45 g/cm³ and (m) the copolymer contains less than 350 ppm of inorganicresidues.
 6. A novel copolymer according to claim 5, characterised inthat its molecular structure, determined by carbon 13 nuclear magneticresonance (NMR) is such that the units derived from comonomers (b) and(c) are distributed along the copolymer chain, at least 95% of theseunits being completely isolated from each other, separated by more thanone unit derived from ethylene.
 7. A process for the production of afinished object comprising producing said object by injection mouldingthe copolymer according to claim
 5. 8. A process for the production of afinished object comprising producing said object by rotational mouldingthe copolymer according to claim
 5. 9. A process for the production of afinished object comprising producing said object by extrusion formingthe copolymer according to claim
 5. 10. A process for the production ofa finished object comprising producing said object by blowing extrusionof the copolymer according to claim
 5. 11. The copolymer of claim 5,wherein in (c), the weight ratio of comonomer (c) to comonomer (b) isbetween 0.25 and 1.